The report by the Equal Opportunities Commission earlier this week, of 1,000 cases of unfair pregnancy dismissal claims lodged in industrial tribunals every year, cannot but bring a groan of recognition to any mother. It's not that our swelling stomachs immediately earned us our P45s, but we've all sensed how our relationship with our employers shifts - sometimes imperceptibly, but always decisively - as soon as the secret vomiting in the toilet ends and the pregnancy is official.

The EOC describes the 1,000 figure as the "tip of the iceberg", and points to the fact that pregnancy and maternity related employment issues are the most common subject of calls to their helpline. Given that women must make a claim within three months of being dismissed, the true figure of pregnancy discrimination is likely to be much higher; a woman who is struggling with the late stages of carrying a child, giving birth or caring for the new baby, is not best placed to launch claims against her former employer for dismissing her.

So we have no real idea of how widespread the problem is. Hence the EOC's decision last year to launch its general formal investigation, of which this report is the opening shot. What the EOC must investigate is why the laws - it is illegal to discriminate against a pregnant woman or give her less favourable treatment, let alone fire her - don't always work.

Part of the problem lies in that decisive shift in the employment contract. Once pregnant, a woman has another obvious alternative focus to her life apart from work, and in a working culture built around a male norm where work is the focus and total availability is presumed, this is perceived as likely to lead to less commitment to the job. Research shows that pregnant women and mothers are acutely aware of how they are regarded by employers as a burden, rather than fulfilling a vital social role of providing the next generation.

Employers manage, to a greater or lesser extent, to conceal their hostility to the process of procreation. But scratch the surface and you find a latent animosity, fuelled by extraordinary ignorance. The manager of a medium-sized company complained about women getting pregnant and costing a lot in maternity pay: he didn't know that the state pays statutory maternity pay.

Once, the announcement of a pregnancy was the occasion for congratulations. Now, it is accompanied by anxious conversations about childcare and the "what are you going to do?" question, which only ever means one thing: how are you going to adjust your working life to fit in a child? "Trouble ahead," ponders every working woman as she begins, on top of the ante-natal tests, nausea and anxiety-inducing baby books, the task of negotiating a deal on part-time flexible working.

Meanwhile, her male colleagues' attitudes change. Once a competitor, she's now slipping inexorably down the "mommy track" and, while there's considerable relief, it's also mixed with a degree of envious resentment. The big bump makes men pretty uncomfortable; they can't wait to see you gone, and at the same time, they're wishing they could have what they like to describe as "time off".

The EOC's investigation may come up with some concrete ideas of how to improve things - extending the time limit is an obvious one. But the nub of this problem probably lies beyond the reach of legislation and industrial tribunals in the working culture of organisations. The biggest challenge is to change the pernicious, outdated norm of what constitutes employee commitment and, crucially, to dismantle time as the crucial measure.

That's also the target of the TUC's campaign on working hours, It's About Time. As part of that campaign, it has designated today Work Your Proper Hours Day, in which, in order to counteract the effect of unpaid overtime, workers are being encouraged to go home on time.

In pessimistic moments, my concern is that the intensification of competition feeds through into neurotic, bullying organisations in which unpaid overtime is routinely expected. They are also places where the weak get weeded out - older workers who can't keep up the pace, those prone to ill-health and, of course, the pregnant - and are relegated to the marginal, less well-paid jobs.

At my more optimistic, I think that this is the transition generation and that the pace of change is so rapid it's not surprising that it's still bumpy. Thirty years ago when a woman got pregnant, the assumption was that she would be leaving her job. We've come a long way; now 45% of the workforce are women and they're getting 80% of the new jobs being created; they are no longer just accessories to the labour market, but the linchpin.

The employment rate of women with small children has shot up since the early 90s, and is set to continue rising. Working culture is changing and employers can't count on obsessive, workaholic males; they have to adapt to a new type of workforce where mothers - and fathers - are combining work and care. So remember, go home on time today.